Pd(II) Complexes with Chelating Phosphinoferrocene Diaminocarbene Ligands: Synthesis, Characterization, and Catalytic Use in Pd-Catalyzed Borylation of Aryl Bromides

We developed a novel, straightforward
route toward Pd­(II)-aminocarbene
complexes bearing a P-chelating phosphinoferrocenyl substituent based
on a three-component reaction of 1′-(diphenylphosphino)-1-isocyanoferrocene
(1) with [PdCl2(cod)] (cod = cycloocta-1,5-diene)
and nucleophilic amines. Depending on the type of the amine, the reaction
produced acyclic diaminocarbenes and their saturated (imidazolin-2-ylidene)
and unsaturated (imidazol-2-ylidene) cyclic counterparts (NHCs). Using
(S)-2-(chloromethyl)­pyrrolidine as the nucleophile,
this method afforded a separable pair of stable diastereomeric bicyclic
imidazolin-2-ylidene carbenes with different configurations of the
planar-chiral ferrocene unit. The prepared P-chelating carbenes were
characterized using spectroscopic methods, X-ray crystallography,
and DFT methods. The last were used to explain the formation of isomeric
open diaminocarbenes featuring NHR groups at the wing-tip position,
trends in Pd–Cl bond lengths reflecting similar trans influences of the particular carbene and phosphine donors, and the
results from cyclic voltammetric measurements. Furthermore, the carbenes
were used as defined (pre)­catalysts in Miyaura borylation of aryl
bromides with bis­(pinacolato)­diboron. When applying the optimized
catalytic system (1 mol % Pd catalyst, KOAc as the base, 2-propanol,
85 °C), this reaction produced a range of simple and substituted
arylboronate pinacol esters in high yield and without biaryl side
products.